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Vacuum system for the semi-automatic degassing of solvents
Short communications ~a,ama,
1969, Vol. 16, pp. 619 tea620. Pergamon Press.
619
Printed in Northern Ireland
Vacuum system for the semi-automatic
degas&g
of solvents
(Received 19 November 1968. Accepted 18 December 1968) THE need to degas solvents arises from the fact that oxygen dissolved in the solvent at normal temperatures is sufficient to disturb rn~u~rnen~ of transient species produced in some phot~he~~ and electrochemical experiments.l Two methods have generally been used for degassing solutions, both of which have disadvantages. The first is bubbling a suitable inert gas, usually nitrogen, through the solution and than sealing off the solution on a vacuum line with or without a small number of “freeze-thaw” cycles. The second method uses repeated “freeze-thaw” cycles until the pressure over the frozen solution reaches an appropriate low value, usually 1O-5-1O-8 mbar. The primary disadvantage of the first method is that the amount of oxygen remaining in the solution after the treatment remains ~0~. Thus inconsistencies may develop when oxygen-sensitive systems are under study. The main disadvantage of the second technique is the large amount of time that must be devoted to the degassing of individual reaction mixtures to obtain the desired consistency of results.8 In order to overcome both of these disadvantages a procedure based on the “freeze-thaw” method has been developed for degassing large volumes of solvent. EXPERIMENTAL The apparatus is shown in Fig. 1. A and 3 are Teflon vacuum stop-cocks; iC is a 1000~ml inline trap which contains liquid nitrogen; D is an O-ring seal ; E contains the liquid to be degassed (about 100 ml).
FIG. I.-Apparatus
used for degassing solvents.
The entire apparatus is attached to a vacuum manifold above stop-cock A. Operationof the system is as follows. With the trap C empty and B closed, A is opened and the space between the two stopcocks is evacuated. While this is being done the solvent in E is being frozen as in the normal freezethaw cycle. With the solvent frozen, B is opened and the whole system is allowed to pump down to a
620
Short communications
suitable pressure. With the liquid in E still frozen, C is filled with liquid nitrogen. With A and B remaining open, the liquid in E is slowly allowed to warm up to room temperature. As the liquid warms,enough dissolved gas is given off to raise the pressure to about 1 ybar. When the liquid is warm enough vapour will begin to condense on the bottom of C while almost all of the dissolved gases will pass through to the pumps. Eventually, a sufficient amount of liquid will condense on the trap so that it will start dripping down to the bulb E. An equilibrium will be set up between the condensation on C and the flow back to E. During this time the pressure in the system as a whole will continue to decrease until the vacuum gauge reading is between 1 and 6 nbar while the degassing apparatus is open to the vacuum pumps. The only precaution that must be taken during the process is that the level of the coolant in the trap does not fall sufficiently to allow escape of the liquid being degassed (refill about every 30 min). Since the liquid flowing back to E is quite cold it is also necessary to heat the system between C and B as well as to heat the liquid in E with warm air at suitable intervals (about 30 min) in order to speed up the circulation between C and E. When the cycle is finished A is closed and the liquid in E is frozen as the coolant in Cis removed (best with a stream of nitrogen gas). B is then closed and the liquid can be stored in E until it is to be used, at which time it can be distilled into a cell which can be attached to another portion of the vacuum system. A similar although less elaborate system has been proposed for degassing individual small samples.* RESULTS The system described has been used to degas methylcyclohexane, benzene and isopropanol. Excellent results have been obtained for all of these. Maintenance of the degassing cycle for from 2 to 3 hr leads to a pressure over the frozen solvent of less than 10 nbar. Continuing the cycle for another hour leads to pressures over the frozen liquids of less than about 6 nbar. This system has the further advantage that the bulb E can be removed with the liquid under vacuum and stored until needed. If storage 7s prolonged, the degassing may have to be re‘peated but will be much shorter in duration since the liauid will not be saturated with dissolved gases. Denendine on the care used during degassing, liquid losses can be held to considerably less thak 10 %dur&g any--. Occasionally, when a large volume of liquid was being degassed, so much would condense on the bottom of the trap that some would escape into the system. This could be easily controlled by noting any large pressure increases, shutting stop-cock A for a moment and heating the area round the bottom of the frap with warm air. The reproducibility of the process is illustrated by the lifetime of the triplet state of phenazine has a value of approximately 50 ,usec both with a number of samples of methylcyclohexane degassed as described and with samples completely degassed by the old, much less efficient, “freeze-thaw” method. Liquids to be degassed should be tested in case they attack the O-ring seals or taps. Acknowledgement-The authors would like to thank Dr. E. W. Abrahamson for his continual encouragement and support. The apparatus was designed for research sponsored by the Atomic Energy Commission, Contract No. At-l&1-904. JOHN R. WIESENFELD STeveN M. JAPAR
Department of Chemistry Cape Western Reserve University Cleveland, Ohio 44106, U.S.A. Summary_---A semi-automatic system capable large amounts of solvent is described.
of quickly degassing
Zusammenfassung-Ein halbautomatisches System wird beschrieben, mit dem man schnell grol3e Lasungsmittelmengen entgasen kann. R&m&-On rapidement
dBcrit un syst&me semi-automatique de grandes quantitb de solvant.
capablede dB gazer
REFERENCES 1. G. Porter and M. Windsor, Proc. Roy. Sot. London, 1958, A245,238; L. Meites in I. M. Kolthoff, P. J. Elving and E. B. Sandell, Treatise on Analytical Chemistry, Part 1, Vol. 4, p. 2358. Interscience, New York, 1963. 2. W. M. Moreau, T. A. Tyler and K. Weiss, J. Chem. Educ., 1966, 43,435. 3. R. E. Rondeau, ibid., 1967,44,530.
1969, Vol. 16, pp. 619 tea620. Pergamon Press.
619
Printed in Northern Ireland
Vacuum system for the semi-automatic
degas&g
of solvents
(Received 19 November 1968. Accepted 18 December 1968) THE need to degas solvents arises from the fact that oxygen dissolved in the solvent at normal temperatures is sufficient to disturb rn~u~rnen~ of transient species produced in some phot~he~~ and electrochemical experiments.l Two methods have generally been used for degassing solutions, both of which have disadvantages. The first is bubbling a suitable inert gas, usually nitrogen, through the solution and than sealing off the solution on a vacuum line with or without a small number of “freeze-thaw” cycles. The second method uses repeated “freeze-thaw” cycles until the pressure over the frozen solution reaches an appropriate low value, usually 1O-5-1O-8 mbar. The primary disadvantage of the first method is that the amount of oxygen remaining in the solution after the treatment remains ~0~. Thus inconsistencies may develop when oxygen-sensitive systems are under study. The main disadvantage of the second technique is the large amount of time that must be devoted to the degassing of individual reaction mixtures to obtain the desired consistency of results.8 In order to overcome both of these disadvantages a procedure based on the “freeze-thaw” method has been developed for degassing large volumes of solvent. EXPERIMENTAL The apparatus is shown in Fig. 1. A and 3 are Teflon vacuum stop-cocks; iC is a 1000~ml inline trap which contains liquid nitrogen; D is an O-ring seal ; E contains the liquid to be degassed (about 100 ml).
FIG. I.-Apparatus
used for degassing solvents.
The entire apparatus is attached to a vacuum manifold above stop-cock A. Operationof the system is as follows. With the trap C empty and B closed, A is opened and the space between the two stopcocks is evacuated. While this is being done the solvent in E is being frozen as in the normal freezethaw cycle. With the solvent frozen, B is opened and the whole system is allowed to pump down to a
620
Short communications
suitable pressure. With the liquid in E still frozen, C is filled with liquid nitrogen. With A and B remaining open, the liquid in E is slowly allowed to warm up to room temperature. As the liquid warms,enough dissolved gas is given off to raise the pressure to about 1 ybar. When the liquid is warm enough vapour will begin to condense on the bottom of C while almost all of the dissolved gases will pass through to the pumps. Eventually, a sufficient amount of liquid will condense on the trap so that it will start dripping down to the bulb E. An equilibrium will be set up between the condensation on C and the flow back to E. During this time the pressure in the system as a whole will continue to decrease until the vacuum gauge reading is between 1 and 6 nbar while the degassing apparatus is open to the vacuum pumps. The only precaution that must be taken during the process is that the level of the coolant in the trap does not fall sufficiently to allow escape of the liquid being degassed (refill about every 30 min). Since the liquid flowing back to E is quite cold it is also necessary to heat the system between C and B as well as to heat the liquid in E with warm air at suitable intervals (about 30 min) in order to speed up the circulation between C and E. When the cycle is finished A is closed and the liquid in E is frozen as the coolant in Cis removed (best with a stream of nitrogen gas). B is then closed and the liquid can be stored in E until it is to be used, at which time it can be distilled into a cell which can be attached to another portion of the vacuum system. A similar although less elaborate system has been proposed for degassing individual small samples.* RESULTS The system described has been used to degas methylcyclohexane, benzene and isopropanol. Excellent results have been obtained for all of these. Maintenance of the degassing cycle for from 2 to 3 hr leads to a pressure over the frozen solvent of less than 10 nbar. Continuing the cycle for another hour leads to pressures over the frozen liquids of less than about 6 nbar. This system has the further advantage that the bulb E can be removed with the liquid under vacuum and stored until needed. If storage 7s prolonged, the degassing may have to be re‘peated but will be much shorter in duration since the liauid will not be saturated with dissolved gases. Denendine on the care used during degassing, liquid losses can be held to considerably less thak 10 %dur&g any--. Occasionally, when a large volume of liquid was being degassed, so much would condense on the bottom of the trap that some would escape into the system. This could be easily controlled by noting any large pressure increases, shutting stop-cock A for a moment and heating the area round the bottom of the frap with warm air. The reproducibility of the process is illustrated by the lifetime of the triplet state of phenazine has a value of approximately 50 ,usec both with a number of samples of methylcyclohexane degassed as described and with samples completely degassed by the old, much less efficient, “freeze-thaw” method. Liquids to be degassed should be tested in case they attack the O-ring seals or taps. Acknowledgement-The authors would like to thank Dr. E. W. Abrahamson for his continual encouragement and support. The apparatus was designed for research sponsored by the Atomic Energy Commission, Contract No. At-l&1-904. JOHN R. WIESENFELD STeveN M. JAPAR
Department of Chemistry Cape Western Reserve University Cleveland, Ohio 44106, U.S.A. Summary_---A semi-automatic system capable large amounts of solvent is described.
of quickly degassing
Zusammenfassung-Ein halbautomatisches System wird beschrieben, mit dem man schnell grol3e Lasungsmittelmengen entgasen kann. R&m&-On rapidement
dBcrit un syst&me semi-automatique de grandes quantitb de solvant.
capablede dB gazer
REFERENCES 1. G. Porter and M. Windsor, Proc. Roy. Sot. London, 1958, A245,238; L. Meites in I. M. Kolthoff, P. J. Elving and E. B. Sandell, Treatise on Analytical Chemistry, Part 1, Vol. 4, p. 2358. Interscience, New York, 1963. 2. W. M. Moreau, T. A. Tyler and K. Weiss, J. Chem. Educ., 1966, 43,435. 3. R. E. Rondeau, ibid., 1967,44,530.